1. Field of the Invention
The present invention relates to an electrostatic image developing toner to be used for developing electrostatic images in an electrophotographic copying machines, laser beam printers and electrostatic recording apparatuses wherein an image is formed utilizing an electrophotographic method or an electrostatic recording method, and a method for forming an image using the electrostatic image developing toner.
2. Description of the Related Art
In copying machines for copying an original, printers for recording output of a computer including a personal computer, or printers for facsimile receiving apparatuses, an electrophotographic method or an electrostatic recording method has long been popularly employed for obtaining a copied or recorded image. Typical examples of the copying machine or printer employing this electrophotographic method or the electrostatic recording method include an electrophotographic copying machine, a laser beam printer, a printer using a liquid crystal array and an electrostatic printer. In the electrophotographic method or the electrostatic recording method, an electrostatic latent image (electrostatic charge image) is formed on an electrostatic image carrier such as an electrophotographic photoreceptor or an electrostatic recording medium through various means, and is developed with a developer. The resultant toner image is transferred, if necessary, to a transfer medium such as paper, then fixed by heating, applying pressure, applying heat and pressure or with a solvent vapor to thereby obtain a final toner image. On the other hand, toner particles not transferred and remaining on the electrostatic image carrier are removed by a cleaning means. By repeating the above-described steps, copied products or recorded products can be obtained in sequence.
As a method for developing the electrostatic image, there have been known a method of using a liquid developer comprising a fine toner dispersed in an electrically insulating liquid; a method of using a powdery toner comprising a colorant and, if necessary, a magnetic substance dispersed in a binder resin together with a particulate carrier; and a method of conducting development using a magnetic toner comprising a magnetic substance dispersed in a binder resin and not using a particulate carrier. Of these methods, dry developing methods using a powdery toner or a magnetic toner have been mainly employed in recent years.
Incidentally, speeding up of electrophotographic copying machines and laser beam printers have recently been required with the progress of reduction in size and personalization and, in addition, it has also been required to reduce necessary energy. Therefore, with these apparatus, various improvements have been attempted to obtain a highly reliable and high-quality image at a high speed with a low energy for a long period of time through a mechanism as simple as possible. Also, in company with such improvements of the apparatus, various improvements have been attempted with respect to toners to be used for development.
For example, as an apparatus for fixing a toner image, there have been widely employed a hot-pressing fixing apparatus using a heating roller and a hot-pressing fixing apparatus wherein a roll-like or continuous heat-resistant film, so-called fixing belt, through which a heating member and the developing surface of a transfer member such as paper is faced each other, and heating is conducted by the heating member, the fixing belt and the transfer member are conveyed in this integrated state while pressing the transfer member from back side thereof using a press roller to thereby conduct hot-pressing fixation. In these methods, the heating roller or the fixing belt directly contacts with a toner image upon fixing of the toner image, and hence conduction of heat to the toner can be realized with such a good efficiency that the toner can be smoothly molten with a low energy at a high speed. In these methods, however, the molten toner comes into direct contact with the heating roller or the fixing belt. Therefore there is involved a problem of staining a transfer member such as paper due to so-called offset phenomenon. That is, a part of the molten toner on a transfer member transfers to and deposits on the surface of the heating roller or the fixing belt, and when the heating roller or the fixing belt again contacts with the transfer member, the toner having transferred and deposited on the heating roller or the like transfers again to the transfer member or, in the case where there exists no transfer member, the toner having transferred to and deposited on the heating roller or the like transfers to a pressing roll and, when a next transfer member passes through the fixing apparatus, it deposits on the back side of the transfer member.
On the other hand, in view of fixing the toner at a low temperature, it is effective to reduce a softening temperature (Tm) of a toner binder resin. However, reduction of Tm generally causes reduction of the glass transition temperature (Tg) of the toner, thus it is known that so-called toner blocking of forming a block of toner during storage or the offset phenomenon of toner upon fixing is liable to take place. This is one of the causes that inhibit to reduce the fixing temperature as one desires. As a technique for simultaneously obtaining the low-temperature fixability and blocking resistance or offset resistance, there has been proposed a technique of using a polyester-based resin which shows a comparatively low fixing temperature even when Tm or Tg is high (Japanese Unexamined Patent Publication No. 1952/1981). However, this technique fails to satisfy simultaneously and sufficiently requirements for low-temperature fixability, blocking resistance and offset resistance.
As another technique, there has been proposed to use a resin composed of a lower molecular component and a higher molecular component. For example, Japanese Unexamined Patent Publication No. 16144/1981 describes to use a styrene-based copolymer mixture showing a chromatogram obtained by gel permeation chromatography (GPC) wherein at least one maximum value of the molecular weight exist in each of a molecular weight region of from 103 to 8xc3x97104 and a molecular weight region of from 105 to 2xc3x97106; Japanese Unexamined Patent Publication No. 229158/1987 describes to use a polyester having 5 to 25% by weight of chloroform-insoluble component; Japanese Unexamined Patent Publication No. 168264/1990 describes to use a carboxylic acid group-containing vinyl polymer having a weight-average molecular weight of 1xc3x97103 to 2xc3x97104 and a ratio of weight-average molecular weight/number-average molecular weight of 3 or less and a carboxylic acid group-containing vinyl polymer having a weight-average molecular weight of 1.5xc3x97105 to 8xc3x97105 and a ratio of weight-average molecular weight/number-average molecular weight of 1.5 or more; Japanese Unexamined Patent Publication No. 294867/1991 describes to use a styrene-acrylic-based copolymer having a molecular weight distribution showing a gel permeation chromatogram wherein a maximum value of the molecular weight exists in each of a molecular weight region of 1xc3x97105 or more and a molecular weight region of from 500 to 2xc3x97104, and wherein a minimum value of the molecular weight exists between the two maximum values, and a ratio of the sum of the areas of two peaks respectively containing the two maximum values to the area of the valley under a tangential line common to the two peaks and containing the minimum value is 0.30 or less; and Japanese Unexamined Patent Publication No. 297629/1993 describes to use a binder resin which has a molecular weight distribution measured by GPC wherein at least one peak exists in a molecular weight region of from 3xc3x97103 to 5xc3x97104 and at least one peak in a molecular weight region of 105 or more, which contains 50% or more components of 105 or less in molecular weight, and which shows specific properties in a DSC curve obtained by measuring with a differential scanning calorimeter. With these binder resins, the low-molecular components are considered to contribute to the low temperature fixability, whereas the high-molecular components are considered to contribute to the blocking resistance or the offset resistance. With these known binder resins, however, there is involved a problem that, as the average molecular weight of the low-molecular component is decreased in order to obtain fixability at a lower temperature, the styrene-based resin having conventionally been widely used as binder resin becomes liquid or undergoes such a decrease in the glass transition point (Tg) that there results a poor storage stability. Thus, in actual fact, when the maximum molecular weight value of the low-molecular component becomes about 2,500 or less, such resin cannot be used as a binder resin for a toner, and hence there have been no binder resins for a toner which show a sufficient fixability at a lower temperature.
Further, in order to prevent the offset phenomenon of a toner, it has been proposed to incorporate in the toner itself a releasant that will be molten by heating upon fixation and come out of the toner to prevent the offset phenomenon. As such releasant, there have been proposed a number of waxes such as low molecular weight polyethylene, low molecular weight polypropylene, hydrocarbon-based waxes, natural waxes, and modified waxes thereof.
As to the techniques for preventing the offset phenomenon of a toner by incorporating the waxes or modified waxes in the toner and as to the waxes or modified waxes to be incorporated in the toner, various descriptions are given in, for example, Japanese Examined Patent Publication No. 3304/1977, Japanese Examined Patent Publication No. 3305/1977, Japanese Examined Patent Publication No. 52574/1982, Japanese Unexamined Patent Publication No. 217366/1985, Japanese Unexamined Patent Publication No. 252360/1985, Japanese Unexamined Patent Publication No. 252361/1985, Japanese Unexamined Patent Publication No. 138259/1986, Japanese Unexamined Patent Publication No. 94062/1986, Japanese Unexamined Patent Publication No. 14166/1987, Japanese Unexamined Patent Publication No. 195683/1987, Japanese Unexamined Patent Publication No. 109359/1989, Japanese Unexamined Patent Publication No. 79860/1990, Japanese Unexamined Patent Publication No. 50559/1991, Japanese Unexamined Patent Publication No. 204185/1993, and Japanese Unexamined Patent Publication No. 75422/1994. However, these waxes to be added to a toner for imparting releasing properties are essentially poor in compatibility with the binder resins and are difficult to uniformly disperse in a toner. In addition, in case where a large amount of the wax is added to the toner in order to prevent the offset phenomenon, the wax is unevenly distributed and, upon pulverizing the toner, the wax is liberated, and the thus liberated wax in turn causes filming on the photoreceptor, developing sleeve, carrier particles, etc., thus causing deterioration of a developed image, and problems such as reduction of fluidity of the toner and blocking of the toner.
On the other hand, an attempt has long been made to use a petroleum resin as a binder resin component of a toner for electrophotography. For example, Japanese Unexamined Patent Publication No. 121049/1981 proposes a pressure-fixable toner containing a C5 to C6 aliphatic petroleum resin; Japanese Unexamined Patent Publication No. 26950/1985 proposes a pressure-fixable toner containing a petroleum resin composed of C5 aliphatic hydrocarbons; and Japanese Unexamined Patent Publication Nos. 263951/1985 and 2161/19861 propose a pressure-fixable toner containing a petroleum resin having a softening point of 160xc2x0 C. or lower than that. These toners are proposed noting the generally soft properties of the petroleum resins. It is further known to use the petroleum resin as a part of a binder resin for a toner to be fixed according to the heat-fixing method. Examples thereof are described in, for example, Japanese Unexamined Patent Publication No. 99740/1975, Japanese Unexamined Patent Publication No. 99741/1975, Japanese Unexamined Patent Publication No. 99742/1975, Japanese Unexamined Patent Publication No. 257868/1992, Japanese Unexamined Patent Publication No. 278658/1996, Japanese Unexamined Patent Publication No. 52611/1999, Japanese Unexamined Patent Publication No. 52612/1999, Japanese Unexamined Patent Publication No. 52614/1999, Japanese Unexamined Patent Publication No. 52615/1999, Japanese Unexamined Patent Publication No. 52616/1999, and Japanese Unexamined Patent Publication No. 5214/2001. Many of the binder resins described in these publications use the petroleum resin as an auxiliary component of the toner binder resin, and there are no binder resins which sufficiently satisfy the requirements for low-temperature fixability, blocking resistance, offset resistance, and pulverizability upon production of a toner.
Thus, there have been desired an electrostatic image developing toner that permits good fixation at a lower temperature than with conventional toners, shows an excellent blocking resistance and offset resistance, and does not involve the problem with respect to pulverizability upon production of a toner. In addition, in order to save sources and reduce copying cost, there have also been demanded a toner that can provide a toner image with a sufficient density even when used in a small amount, and an image-forming method using such toner.
An object of the invention is to provide an electrostatic image developing toner capable of solving the above-described problems.
That is, an object of the invention is to provide an electrostatic image developing toner, which has an excellent low-temperature fixability and an excellent offset resistance.
Another object of the invention is to provide an electrostatic image developing toner, which has an excellent low-temperature fixability and an excellent offset resistance and which, when stored or left for a long period of time, does not suffer deterioration.
A further object of the invention is to provide an electrostatic image developing toner which has the above described various properties at the same time and has an excellent pulverizability.
A still further object of the invention is to provide a method for conducting fixation at a low temperature using an electrostatic image developing toner having the above-described properties and forming a copied or recorded image without causing offset phenomenon.
A yet further object of the invention is to provide an electrostatic image developing toner that can provide a copied or recorded product having an enough density even when it is used only in a small amount.
As a result of intensive investigations to solve the above described problems with the conventional toners, the inventors have found that a copied or recorded product with a desired density can be obtained even when a toner is used only in a small amount with an excellent low temperature fixability without causing blocking or offset phenomenon of the toner, as has conventionally been desired, by using a petroleum resin as a low molecular component of a toner binder resin and, as a high molecular component of the toner binder resin, a styrene-based copolymer or a styrene-based homopolymer, with adjusting the ranges of molecular weight where the maximum values of the molecular weight of the petroleum resin and that of the styrene-based copolymer or the styrene-based homopolymer in a chromatogram of the toner binder resin measured according to gel permeation chromatography (GPC) exist in a specific range and adjusting the ratio of the amounts thereof to specific ranges, and, further using a wax having a low melt viscosity at 140xc2x0 C. as a releasant, and that such toner has an excellent pulverizability upon production of the toner, thus having achieved the present invention based on the findings.
That is, the present invention provides an electrostatic image developing toner containing at least a binder resin and a releasant, wherein the binder resin comprises a petroleum resin and a styrene-based copolymer or styrene-based homopolymer in an amount of 5:5 to 7:3 by weight in terms of the ratio of the petroleum resin:styrene-based copolymer or styrene-based homopolymer and has the maximum molecular weight value (PL) based on the petroleum resin in a range of from 5xc3x97102 to 2xc3x97103 and the maximum molecular weight value (PH) based on the styrene-based copolymer or styrene-based homopolymer in a range of from 1xc3x97105 to 4.5xc3x97105 in the chromatogram obtained by measuring the binder resin through gel permeation chromatography and, in addition, the releasant is a wax having a melt viscosity at 140xc2x0 C. of 102 mPa/s (cp) or less and is added in an amount of 1 to 7 parts by weight per 100 parts by weight of the toner.
The invention also provides an image-forming method, which involves a step of forming an electrostatic image by forming an electrostatic latent image on an electrostatic latent image carrier; an image-forming step of developing the electrostatic latent image using the above-described electrostatic image developing toner; a transferring step of transferring the toner image on a transfer member; and a step of heat-fixing by heating the toner image transferred onto the transfer member.
In the invention, it is preferred that the petroleum resin has a glass transition point Tg of 35 to 55xc2x0 C., and the styrene-based copolymer has a glass transition point Tg of 50 to 75xc2x0 C.
In addition, it is preferred in the invention that HPL:HM:HPH=25 to 65:1:20 to 40, wherein HPL represents the height of the maximum molecular weight value PL based on the petroleum resin in the chromatogram obtained by measuring through gel permeation chromatography, HPH represents the height of the maximum molecular weight value PH based on the styrene-based copolymer or the styrene-based homopolymer in the chromatogram obtained by measuring through gel permeation chromatography, and HM represents the minimum height between the two maximum values.
In the invention, the wax preferably shows a weight loss of less than 1.5% when subjected to measurement in a thermal analysis apparatus TGA at 200xc2x0 C.
In the invention, the low molecular component of petroleum resin mainly governs the fixability, whereas the high molecular component of styrene-based copolymer or styrene-based homopolymer mainly governs the offset-resistant properties and the anti-block properties. The fixability, offset-resistant properties and anti-block properties can be well balanced by properly mixing these components. Also, releasing properties of the toner are more improved by the releasant to obtain a fixed image without offset phenomenon.